The present invention relates to improvements in methods and apparatus for transferring a flexible web from reel to reel of a web/reel assembly, and more particularly to tape transport apparatus for tape cassettes and methods for controlling and operating such apparatus. In more detail, the present invention relates to improvements in the method and apparatus disclosed in the commonly assigned prior U.S. Pat. No. 4,172,231 which describes methods and apparatus for accurately determining absolute values of tape position as tape is driven from reel to reel of a tape/reel assembly, and for controlling the apparatus in accordance with such tape position determinations.
The invention of the aforesaid prior application is described therein as useful in transport apparatus for reel/web assemblies the physical parameters of which are known, the physical parameters including the tape length and thickness, and the reel hub diameters. In such a transport apparatus operating with a web/reel assembly having known physical parameters, the position relative to one end of the web of an intermediate portion being transferred from one reel to the other, may be determined by a computational process using mathematical equations, i.e. by following tape position determination algorithms employing constants established by the known physical parameters of that reel/web assembly, and a variable parameter; namely, the ratio of rotational speeds of the two reels as the intermediate web portion is being transferred from one reel to the other, which ratio continuously changes as the web is transferred. It is explained in said prior application that the same algorithms employing the same constants may be followed to determine the position of an intermediate web portion at any stage of transfer of the web; the only input required at any stage is the ratio of the rotational speeds of the reels. By determining tape position in a regularly repeating cycle, using said tape position determination algorithms, and producing each cycle an output signal representing the position determined for the web during that cycle, the output signals may be utilized to monitor the transfer of the web in the apparatus, for example, by displaying the continuously changing position of the web or controlling the operation of the transport apparatus.
The method of said prior application is particularly, although not exclusively, useful in tape transport apparatus for cassettes of tape which are a standard type for example, cassettes providing sixty, ninety and one hundred twenty minutes of playing time known, respectively, as C-60, C-90 and C-120 cassettes. Such cassettes are conventional with two reels and different known lengths and thickness of tape to provide the various lengths of playing time. The method entails first establishing constants for each tape cassette, which constants are used in the position determination algorithms, and storing a set of constants for each tape cassette in, for example, a semiconductor memory unit. In tape transport apparatus under the control of a microprocessor, signals representing the set of constants corresponding to the cassette loaded into the transport apparatus, are recalled from the memory unit, intermediate signals are generated representing the ratio P.sub.1 /P.sub.2 measuring the rotational speeds of the reels, and by circuit means such as the microprocessor under program control a determination is made of tape position in terms of the length (l.sub.1 or l.sub.2) or time (t.sub.1 or t.sub. 2) to the end of the web on either reel of an intermediate portion of the web being transferred from one reel to the other, using position determination algorithms employing the following equations: ##EQU1## L representing the total length of magnetic tape, l.sub.1 representing the used tape, l.sub.2 the unused tape, T representing the ratio of P.sub.1 /P.sub.2 when L=l.sub.2, i.e. at the beginning of the tape. T is thus a constant which characterizes each type of cassette and which can be determined either mathematically or by experiment.
If the two terms of the Equation (I) and (I)' are divided by L, we obtain the following: ##EQU2##
It is also possible to multiply the two terms of Equation II and II' by the duration .tau. of the cassette, of which the usual types may have the values defined above. We then obtain, with ##EQU3##
Among the above Equations, (I) and (I)' represent the length of the magnetic wound on the take-up reel and on the supply reel respectively, i.e. a position determination in terms of linear dimension. Equations (II) and (II)' represent the ratios of the length of the magnetic tape wound on the take-up reel and on the supply reel respectively to its total length, i.e. a position indication in non-dimensional terms, while Equations (III) and (III)' represent the time taken by the magnetic tape to reach the radius R.sub.1 on the take-up reel and the radius R.sub.2 on the supply reel respectively, i.e. a position indication in terms of time, representing the playback time or recording time which has elapsed since the beginning of the magnetic tape or which remains until the end of the magnetic tape respectively.
The sets of constants A, A', A", and B, B', B" for the different types of cassettes are determined statistically, so that account can be taken, with sufficient accuracy, of the very slight variations which may exist between different cassettes of one end of the same type and which result, for example, from production tolerances. Constants for the C-120 tape cassette have been measured as A"=72.2496 and B"=5,9961, as an example of constants for use in web position determination using Equation III'.
Accordingly, the present invention is directed to method and means for monitoring the movement of tape in tape transport apparatus for reel/tape assemblies such as cassettes, using tape position determination algorithms specified in the hereinbefore identified commonly assigned prior patent application, for determining absolute values of tape position as tape is driven from reel to reel. Such tape position determinations are based on numerical constants stored in memory and reel rotational speed ratios measured at intervals.
According to the method disclosed in said prior patent application, the rotational speeds of the reels are represented by pulse streams from pulse generators driven responsive to rotation of each reel. Pulses at the rate of twenty per revolution of each reel are fed to shift registers which accumulate the pulses and reel speed ratios are calculated based on the total number of pulse accumulated in the registers. It was recognized that the accumulated total of pulses in a shift register provided only an approximate representation of the instantaneous rotational speed of one of the reels.
It is one of the principal objects of this invention to provide a more accurate measure of instantaneous rotational speed of each reel to base the calculation of reel speed ratios, which is used in the determination of tape position.
According to this invention, instantaneous reel speed ratios are calculated based on the reel time for each reel to make two revolutions when driven at normal (play or record) speed, and four revolutions when driven at fast (forward or rewind) speed. In the present case, pulse generators on the reel spindles produce eight pulses (hereinafter called "reel pulses") each revolution of the reel, which pulses are counted. Clock pulses generated at a rate determined by an internal clock, illustratively 4 KHz, are clocked into and accumulated in a timing register for each reel for the time period elapsed until either 16 or 32 reel pulses are counted, the content of each of the timing registers then representing the real time for two or four revolutions of each of the reels; instantaneous reel speed ratios are calculated by dividing the contents of the timing registers.
In the system described in the aforesaid prior patent application, the operator designates that the web/reel assembly or cassette is of one known type or of another type, to derive from memory numerical constants for basing the tape position determinations. This manual selection entails certain risks in cases where the operator makes the wrong choice, for example, by operating the key corresponding to type C-90 when a C-60 type cassette has been inserted, either by inadvertence or through ignorance of the type of cassette. This may result in malfunctioning of the apparatus and possible damage to the magnetic tape. For example, the display of the position of the tape may be incorrect; it may be impossible to find a predetermined position on the tape during the search mode; there may occur premature slowingdown with unnecessary increase of forward winding or rewinding times; there may occur a failure to slow down towards the end of the tape and consequently the risk of breaking the tape.
To overcome these problems, another important object of the invention is to provide a method and means for automatically identifying and designating the type of web/reel assembly or cassette in the apparatus.
According to this aspect of the invention, the unknown web/reel assembly or cassette is identified by first recalling from memory predetermined values of constants, which are called I.D. constants, and then proceeding through a cassette identifying process in which a first tape position determination is made based on the recalled numerical constants, the tape is moved a prescribed distance, a second tape position determination is made, the actual prescribed distance is compared with the theoretical distance between the first and and second determined positions and in accordance with that comparison, the unknown cassette is determined to be one of the known types, and thus the unknown cassette is identified. In carrying out the I.D. mode of operation in accordance with the invention, preferably the two positions are determined by operation of a microprocessor under program control using position determination algorithms including one of the equations (I) to (III)' referred to above for computing tape positions employing numerical constants derived from the I.D. constants store, and reel speed ratio is measured as the tape is transferred from reel to reel. The two predetermined positions can be selected in arbitrary manner, and may in particular be fixed by predetermined numbers of revolutions of the reels or pulses supplied by the rotational speed detector or detectors (pulse generators) of one and/or the other of the reels. The process is thus based on measuring the rotational velocity of each of the reels, which velocity is dependent on their characteristics, their state of winding, and the type of tape used.
According to another aspect of the invention, this method for automatically identifying an unknown cassette as one of a plurality of known types, is implemented in tape transport apparatus operated under the control of a controller that includes a central processor unit (CPU) and associated memory units (ROM) for programs and storage of numerical constants representing the cassettes, and constants used in the I.D. mode hereinafter called cassette I.D. constants. Further in accordance with the apparatus aspect of the invention, tape transport apparatus including reel drive motors and motor control circuits therefor, is controlled by the microprocessor type controller to perform the I.D. mode of operation in which the tape is moved from a starting position at normal speed for the first tape position determination, at fast speed to the second position, at normal speed for the second tape position determination, and after the cassette is identified, the tape is returned to the starting position. The microprocessor under program control provides means for determining the first and second tape positions, means for calculating the theoretical difference between the first and second positions, means for measuring the actual difference between the same positions, means for comparing the theoretical and actual differences and in accordance with the comparison confirming that the unknown cassette is of one or of another known type.
Another important object of the invention is to provide a method for carrying out high speed searches for target tape positions, which provides random access to any tape position. While the system of said prior patent application includes a search mode of operation, the present method controls the apparatus to reach a target tape position more accurately and at higher speed than was achievable heretofore.
A related object is to provide a method for monitoring tape movement in accordance with pulses from the reel pulse generator of the apparatus and representing reel movement, using a reel pulse count store correlating tape position and reel pulse counts, the pulse count store providing the number of expected pulses to reach given positions or tape positions corresponding to specified pulse counts.
Another object is to provide method and means for counting pulses representing reel movement, and useful in the search and other operational modes of the apparatus.
In the system described in the prior patent application, a display is operated by the controller to display successive tape positions determined by the controller following several revolutions of both reels. If such display is updated to show the tape position each such determination, the display may jump 6 to 8 seconds at a time and the jumps may be uneven. An important further object of the present invention is to provide a method for operating such a display so that the display is uniformly and smoothly updated to show changes in tape position. For example, where tape position is displayed in terms of time to the end of the tape, it is an objective to operate the display to show changes in tape position of one second, the display being operated by a display clock and the rate of the display clock being synchronized with the actual rate of movement of the tape by comparing the actual tape position as determined each cycle with the tape position shown on the display, and changing the rate of the display clock to eliminate any difference between the actual position and the display position over a prolonged interval, to smooth the operation of the display and synchronize it with tape movement.
Another object of the invention is to provide a tape transport method and apparatus for adjusting the output torque of the take-up reel drive motor to maintain a constant tractive force on the tape throughout the process of transferring tape from the supply reel to the take-up reel, by varying the voltage applied to the take-up motor through multiple step levels approximating a linear change proportional to the quantity of tape wound on the take-up reel.